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| Dissertation / PhD Thesis | FZJ-2014-04916 |
2014
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-89336-983-6
Please use a persistent id in citations: http://hdl.handle.net/2128/7939
Abstract: During the last decades, the research on fundamental magnetic structures, like domain walls, spinwaves and vortices, resulted in a detailed understanding of the magnetization dynamics in ferromagnetic materials, without which the development of modern storage devices would not have been possible. On the pathway to this level of understanding micromagnetic simulations played an important role due to their ability to reproduce experimental results in great detail and, especially, to predict magnetic patterns and their dynamic properties. An example of the predictive power is the research field of vortex dynamics. Another fundamental magnetic structure is the Bloch point, which is particularly complex since the description of the processes and energy terms responsible for its formation lie within the scope of the continuum theory of micromagnetism, but the study of its detailed properties requires a different framework. In terms of topology and concerning the exchange energy density, the Bloch point displays a point singularity in the theory of micromagnetism. Bloch points are not a marginal phenomenon; they play, e.g., an important role as transient structures during the switching of vortex cores and reside inside of the archetypal example of vortex domain walls in solid cylindrical nanowires. In the 1960s, E. Feldtkeller and W. Döring described and characterized Bloch points with the then available methods, yet their dynamics eluded a detailed description, since on one hand a large volume is necessary to stabilize a Bloch point structure and on the other hand an atomistic description of its center is required. To solve this problem we developed a multiscale multimodel simulation framework in the context of this thesis, which is able to detect automatically Bloch points as well as other micromagnetically critical structures. In that simulation kit we apply a classical Heisenberg model to the critical regions, while using the framework of micromagnetism for the remaining sample, which is discretized with finite elements. The program allows not only for a static examination of Bloch points residing in a localized Heisenberg approximated region, but also for dynamic simulations due to its ability to detect regions of interest automatically as well as to track them with the multimodel region. The simulations within this thesis focus on ferromagnetic cylindrical nanowires with vortex domain walls. The simulations describe the depinning field necessary to trigger a propagation of the domain wall with the Bloch point in its center and the impact of the relative orientation of the lattice to the Bloch point propagation direction. In addition, we could identify different propagation patterns of the structure consisting of domain wall and Bloch point. In addition to regimes with a continuous domain wall movement, this thesis highlights and discusses several complex modes of domain wall/Bloch point propagation. In particular, we find a propagation regime in which the Bloch point and domain wall propagate with constant velocity above the minimum spin wave phase velocity. This velocity remains constant within a broad interval of external field strength. Using analytic calculations we could ascribe this maximum velocity, which is a feature of potential interest from a technological perspective, to an intrinsic property of the Bloch point. [...]
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